Even low doses of aspirin may cause bleeding so it really should not be used by people who are unlikely to benefit

Professor Tom Meade

They studied the effect of aspirin in 5,500 men aged between 45 and 69 who were at higher than average chance of developing heart disease.

The researchers at the Wolfson Institute of Preventive Medicine in London found there was a risk of serious bleeding in men with high blood pressure but their chances of having heart problems were significantly reduced.

And even in men with low blood pressure, the benefit does not necessarily outweigh the risk, they said.

The advice to take 75mg a day of aspirin - about quarter of a normal tablet - remains for people who have already had a heart attack.

The project was co-ordinated by the Medical Research Council and part-funded by the British Heart Foundation.

Over a period of seven years, the numbers of heart attacks and strokes among men taking aspirin, another clot-busting drug - warfarin - and placebos were compared.

Reduce

Aspirin was found to reduce the number of heart attacks by 20% - mostly affecting non-fatal incidents and generally affecting only those with low blood pressure, reports the British Medical Journal

Professor Tom Meade, who led the research, said: "Although aspirin does reduce the risk of a first heart attack the effect is not large. Even low doses of aspirin may cause bleeding, which is sometimes serious, so it really should not be used by people who are unlikely to benefit.

"Other studies have also suggested that aspirin should be avoided in those with high blood pressure, at any rate until this has been lowered by treatment. However, most people who have previously had a heart attack should take aspirin as the benefits for them are much greater."

And Professor Charles George, medical director of the British Heart Foundation, added: "This research just goes to show that one cannot assume that just because a drug has an effect in one group of people, others will get the same benefit.

"It also serves to remind us that although we are all familiar with aspirin and probably think of it as completely benign, it is still a drug with the potential to do harm if used inappropriately."